Contents

This document explains the relationship between the new
cable
modem max-cpe n command and the
MAX-CPE parameter specified in the
Data-over-Cable Service Interface Specifications (DOCSIS) configuration file.
The DOCSIS configuration file is loaded into the cable modem, and can be coded
on the uBR7200 series routers running Cisco IOS® Software Release 12.1(2)EC1 or
higher.

This document is not restricted to specific software and hardware
versions.

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

There is a demand from cable customers for the Cable Modem Termination
System (CMTS) to permit more CPEs to connect to the cable modem, despite the
fact that the value in the cable modems configuration file limits the number of
hosts. The change to permit more CPEs to connect to the cable modem was
introduced with
CSCdp52029
(registered customers only)
in
Cisco IOS Software Release12.0(9.5)SC and Cisco IOS Software Release
12.1(1.0.3)EC1 IOS trains.

This demand was created due to the cable modem being unable to keep the
count of CPEs allocated in synch with the CMTS count. When the cable modem
initializes, the only time it can synchronize
MAX-CPE with the CMTS is when it sends the
MAX-CPE count to the CMTS in the registration
request flow.

If the cable modem is reloaded, there is no flow to the CMTS to inform
it to reduce the CPE's allocated count to zero. The cable modem, however, will
start again from zero. If a new CPE behind the cable modem requests to come
online, the cable modem will permit it to as its
MAX-CPE count is now zero. The CMTS with a non
zero MAX-CPE count , however, will fail the
request, and the following message appears on the CMTS:

This is intensified by the fact that, if unspecified, the default
MAX-CPE entry in the DOCSIS configuration file
is set to one.

An example of this is when the cable modem informs the CMTS that it has
a MAX-CPE count of one in its registration
request. A cable customer wants the functionality of power cycling the cable
modem to allow their installer's laptop to be removed, and the end user's PC
put back on the cable modem. This will fail because. After the reload the cable
modem will have the MAX-CPE count set to zero,
however, the CMTS will still remember the installer's laptop entry, and which
equals the MAX-CPE count of one.

There is a workaround available by issuing the
clear
cable host x.x.x.x command, where
x.x.x.x equals either the
IP or MAC address of the host to be cleared on the CMTS. This workaround,
however, is unpopular to cable customers.

A new configuration command has been added to the CMTS. To specify a
maximum number of permitted hosts per modem (overriding the
MAX-CPE value in the cable modem configuration
file), issue the
cable
modem max-cpe ncommand in cable
interface configuration mode.
n equals 1 to 255.

The CMTS enables up to
n number of hosts for a
cable modem. When set to unlimited, or
n is larger than the
MAX-CPE value in the configuration file of a
cable modem, this command overrides the configuration file value. The DHCP
server controls the number of IP addresses assigned to hosts behind a single
cable modem.

If the cable modem's configuration file specifies a number of hosts
larger than n, the
configuration file-setting will take precedence. It is up to the cable modem to
control the number of active hosts.

By issuing the
cable
modem max-cpe unlimited command, the CMTS will not enforce
a limit on the number of CPEs connected to a single cable modem. With this
setting, it is up to the cable modem to control the maximum number of CPEs, and
up to the DHCP server to control the number of IP addresses assigned to CPEs
behind a single cable modem.

Caution: Use of the cable modem max-cpe unlimited
command, if used without caution, may open a security hole in the system by
enabling denial of service attacks. Specifically, it may enable a user to
obtain a large number of IP addresses, thereby taking down the entire network
after all available IP addresses have been reserved by this single user. It is
therefore recommended that, if the cable modem max-cpe
unlimited command is enabled, the number of IP addresses assigned
to CPEs behind a single cable modem be strictly controlled by the DHCP
server.

By issuing the
no
cable modem max-cpe command, the default setting is
enabled. The MAX-CPE value provided in the
configuration file will be used by the CMTS to limit the number of CPEs
connected to a single cable modem.

The show cable modem detail command displays
the MAX-CPE value as configured in the DOCSIS
configuration file for the cable modem, and if possible, the value configured
when the cable modem max-cpe
n is issued.

Refer to the sample log output below. CMTS is configured for
MAX-CPE equal to four, and
MAX-CPE equal to unlimited.

Note: Although the CMTS, permits the specified number of hosts through
cable modems, the cable modems themselves will still be restricted to allow
only the number of CPEs permitted in the DOCSIS configuration file.